1. Field of the Invention
The present invention relates generally to a motor drive circuit, and more particularly to a motor drive circuit capable of rotatably driving a DC motor in forward and reverse rotation.
2. Description of the Related Art
FIG. 9 is a diagram of a conventional motor drive circuit. The conventional motor drive circuit 300, which is driven by a DC power source 301 connected to terminals Tv11 and Tv12, comprises a first drive transistor QA for controlling a drive current supplied to a first terminal Tm11 of the motor 302 from the DC power source 301; a second drive transistor QB for drawing current from a first terminal Tm11 of the motor 302 and supplying it to a ground; a third drive transistor QC for controlling a drive current supplied to a second terminal Tm12 of the motor 302 from the DC power source 301; a fourth drive transistor QD for drawing current from the second terminal Tm12 of the motor 302; a drive control circuit 303 for controlling movements of the motor 302 by controlling first, second, third and fourth drive transistors QA, QB, QC and QD in response to a first control signal and a second control signal supplied to a first control terminal Tc11 and a second control terminal Tc12; and diodes D11, D12, D13 and D14 for absorbing a positive deflection from a counter electromotive force generated to the motor 302.
The first drive transistor QA comprises a PNP transistor. An emitter of the PNP transistor is connected to a positive terminal of the DC power source 301 and a collector of the PNP transistor is connected to the first terminal Tm11 of the motor 302. A first drive control signal is supplied to a base of the PNP transistor from the drive control circuit 303. The second drive transistor QB comprises an NPN transistor. An emitter of the NPN transistor is connected to a negative terminal of the DC power source 301 and a collector of the NPN transistor is connected to the first terminal Tm11 of the motor 302. A second drive control signal is supplied to a base of the NPN transistor from the drive control circuit 303.
The third drive transistor QC comprises a PNP transistor. The emitter of the PNP transistor is connected to the positive terminal of the DC power source 301 and the collector of the PNP transistor is connected to the second terminal Tm12 of the motor 302. A third drive control signal is supplied to the base of the PNP transistor from the drive control circuit 303. The fourth drive transistor QB comprises an NPN transistor. The emitter of the NPN transistor is connected to the negative terminal of the DC motor 301 and the collector of the NPN transistor is connected to the second terminal Tm12 of the power source 302. A fourth drive control signal is supplied to the base of the NPN transistor from the drive control circuit 303.
The second drive control signal supplied to the base of the second drive transistor QB is an inversion of the third control signal supplied to the base of the third drive transistor QC. The fourth drive control signal supplied to the base of the fourth drive transistor QD is an inversion of the first control signal supplied to the base of the first drive transistor QA.
The first, second, third and fourth drive control signals are generated in response to the first and second drive signals supplied to the first and second control terminals Tc11 and Tc12 by the drive control circuit.
In the conventional motor drive circuit 300, if for example the first control signal supplied to the first control terminal Tc11 is HIGH and the second control signal supplied to the second control terminal Tc12 is LOW, then the first drive transistor QA and the fourth drive transistor QD are ON and the second drive transistor QB and the third drive transistor QC are OFF, a drive current flows from the first terminal Tm11 to the second terminal Tm12 to the motor 302 and the motor rotates in a forward direction.
Conversely, if the first control signal supplied to the first control terminal Tc11 is LOW and the second control signal supplied to the second control terminal Tc12 is HIGH, then the first drive transistor QA and the fourth drive transistor QD are OFF and the second drive transistor QB and the third drive transistor QC are ON, a drive current flows from the second terminal Tm12 to the first terminal Tm11 to the motor 302 and the motor rotates in a reverse direction.
Additionally, if both the first and second control signals supplied to the first and second control terminals Tc11 and Tc12 are HIGH, then the first and third transistors are OFF and the second and fourth transistors are ON, a brake is applied to the motor 302 and the motor 302 generates a counter-electromotive force from the brake.
FIG. 10 shows an equivalent circuit diagram of a motor. The motor 302 has a built-in coil, and so in general can be expressed as an equivalent circuit connecting in series an inductor ML and a load resistor RL as shown in FIG. 10.
Accordingly, when a drive current Im is supplied to the motor 302, a counter-electromotive force is generated by the inductor ML. Specifically, the load of the motor 302 rotates by inertia when the brake is applied, and this rotation of the load generates a reverse voltage which becomes the counter-electromotive force.
If at this point the motor drive circuit is made into an integrated circuit 300, then a negative voltage is generated by the counter-electromotive force arising during this braking at first and second terminals Tm11 and Tm12 connecting the motor 302. The IC forms a plurality of elements including transistors by doping a semiconductor substrate with impurities. Adjacent elements are configured so that a semiconductor layer has a reverse bias polarity. As a result, the motor operates normally with a bias voltage in a predetermined direction.
However, when a negative voltage is generated at first and second terminals Tm11 and Tm12 and a state of the bias of the substrate breaks down, a semiconductor layer between the elements operates as a parasitic element and locks up the device.
As a result, diodes D11, D12, D13 and D14 are provided to absorb the negative voltage generated at both terminals of the motor 302 by operation of the brake.
Specifically, diodes D12 and D14 prevent voltage deflection toward the first and second terminals T11 and T12 during braking and thus prevent the generation of a lock-up state.
If the first and second terminals Tm11 and Tm12 are deflected negatively by the counter-electromotive force arising at the motor 302 when braking the motor 302, then current is supplied from the ground by the diodes D12 and D14 going ON, the negative deflection of the first and second terminals Tm11 and Tm12 of the motor 302 is limited and the generation of a lock-up state caused by the counter-electromotive force is prevented.
FIG. 11 is a diagram of an example of a conventional operating wave form. FIG. 11(A) shows an operating mode of the motor 302 and FIG. 11(B) shows a voltage at a first terminal Tm11 of the motor 302. A negative voltage is generated at the first terminal as shown in FIG. 11(B) when the operating mode of the motor 302 shifts from a state of forward rotation to a state of braking as shown in FIG. 11(A). When negative voltage is generated by the counter-electromotive force of the motor 302, the voltage VF in a forward direction of the diodes D12 and D14 is clamped by diodes D12 and D14 and does not further decline. At this time the voltage VF is clamped by the negative voltage generated by the counter-electromotive force of the motor 302 at a maximum of 1.2 [V].
However, conventional motor drive circuits are configured so that the current generated by the counter-electromotive force is bypassed to the ground by the diodes D12 and D14. Thus a large current flows through these diodes D12 and D14, and it is therefore necessary to make the size of elements such as the diodes D12 and D14 relatively large. As a consequence, when the motor drive circuit is made into an integrated circuit (hereinafter IC chip) the surface area of this IC chip increases.
Additionally, due to variations in the characteristics of the elements during manufacture and assembly it sometimes happens that diodes D12 and D14 alone cannot adequately absorb the negative current created by the counter-electromotive force.